The present invention relates to a method for managing the charging of a battery comprising at least one storage cell and which is employed in cold environments. It additionally covers an electronic system for a battery, and a battery employing such system.
An electrochemical cell or rechargeable cell (these two terms being equivalent) is a device for producing electricity in which chemical energy is converted into electrical energy. The chemical energy is constituted by electrochemically active compounds deposited on at least one face of electrodes arranged in the cell. The electrical energy is produced by electrochemical reactions during discharge of the cell. The electrodes, arranged in a container, are electrically connected to current output terminals to provide electrical continuity between the electrodes and an electrical consumer with which the storage cell is associated.
A battery is generally designed to operate under so-called nominal conditions in other words within given temperature, current and voltage ranges. The use of a battery outside of its nominal temperature range, which is typically between 0° C. and 40° C. can bring about a limitation of its performance or premature ageing. For example, the charge performed at a temperature which is too low can result in an insufficiently charged battery and discharging performed at a temperature which is too low can result in a deterioration of battery performance, for example an incapacity to supply heavy currents. French patent FR-A-2,282,735 teaches a method for charging a battery in which the battery is warmed by supply from a heating circuit when its temperature is below a determined value.
Now, some batteries are employed in cold environments in which the mean ambient temperature is below the minimum temperature of the nominal operating range of the battery. Thus, for example, standby power batteries for telecommunications cabinets located externally of buildings can be exposed to temperatures down to −20° C. or lower during the winter. Similarly, certain portable tools can be used in cold environments. Under such temperature conditions, battery performance can be significantly degraded, or even prove insufficient when called on to do service, thereby rendering the batteries nonoperational.
The batteries are generally kept in a charge state by control electronics which monitor their state of charge (SOC), battery charging being commanded from a main power supply in order to guarantee a state of charge (SOC) approaching 100% when the battery is required to be used.
The problem of battery temperature is particularly sensitive in the phases of charge balancing and maintaining charge since, contrary to the charging phase, these phases bring about little warming up of the battery since they employ small currents. In effect, if we designate battery capacity by C, fast charging is typically performed with a current comprised between C/10 and C whereas balancing and trickle charging take place using a current between C/100 and C/20 and, respectively, C/500 and C/100. For example, for a 10 A.h capacity battery, this means that a current comprised between 1 A and 10 A is employed for the charging phase whereas currents comprised between 100 mA and 500 mA and, respectively, 20 mA and 100 mA are employed for the charge balancing and trickle charging phases. During these balancing and trickle charging phases, the current applied to the battery is insufficient to bring about warming of the battery.
It is consequently necessary to provide an electronic system which allows batteries employed in a cold environment to be maintained within their normal temperature range to ensure they operate correctly when required to provide service.
It is known to position a heater around the battery. This is for example what is taught in U.S. Pat. No. 5,281,792. Such a heater is constituted by a resistive system which dissipates heat energy to the battery by the Joule effect. The heater is positioned around the battery in such a way as to ensure good thermal transfer to the battery.
It is also possible to arrange heating elements between the individual cells of the battery. For example, European patent application EP-A-1,261,065 teaches the use of a thermal cover arranged between the battery cells for circulation of, depending on the need, refrigerating or heating liquid.
The heater or heating cover require an external source of energy and require to be controlled by their own thermostatic system or the battery charging system. The use of such a heating system involves adding a component which increases overall cost of the battery.
There is consequently a need for an economic solution which allows batteries to be used in cold environments.